Higher plants synthesize a diversified array of non-protein amino acids many of which bear marked structural analogy to the amino acids of mammalian metabolism. As a result, these secondary plant constituents are usually highly toxic and exhibit pronounced antimetabolic properties in animals. A particular group of naturally occurring non-protein amino acids, namely L-canavanine, L-canaline, and O-ureido-L-homoserine, are structurally related to the intermedates of the Krebs-Henseleit ornithine-urea cycle of animals. This group of non-protein amino acids will be utilzed as a model for understanding the biochemical basis for the toxicity and mode of action of these toxic amino acids. Emphasis will be placed on improving the utility of these compounds in fundamental biochemical studies.